Source: Buffer solution diagram — Wikimedia Commons (CC BY-SA)

Diagram labels and annotations:

BUFFER SYSTEM (Weak Acid + Conjugate Salt)
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Example: $CH_{3}COOH$ / $CH_{3}COO^{-}$ $Na^{+}$

         Ka = $1.8 \times 10^{-5}$; pKa = 4.74

[Region A] Add strong acid ($H^{+}$):
  $H^{+}$ + $CH_{3}COO^{-}$ ──► $CH_{3}COOH$
  (conjugate base neutralises added $H^{+}$; pH stays near 4.74)

[Region B] Buffer zone (pH = pKa ± 1):
  pH = pKa + log([salt]/[acid])
  At [salt] = [acid]: pH = pKa = 4.74  ← Most effective buffer point

[Region C] Add strong base ($OH^{-}$):
  $OH^{-}$ + $CH_{3}COOH$ ──► $CH_{3}COO^{-}$ + $H_{2}O$
  (weak acid neutralises added $OH^{-}$; pH stays near 4.74)

[Capacity limit]: When all of one component is consumed,
  buffer capacity is exceeded and pH changes sharply.

Key labeling concepts:

• Acidic buffer: weak acid + conjugate base salt (e.g., SMILES: CC(=O)O (acetic acid) + $CH_{3}COONa$)
• Basic buffer: weak base + conjugate acid salt (e.g., $NH_{4}OH$ + $NH_{4}Cl$)
• Buffer range: pH = pKa ± 1